1. Field of the Invention
The present invention relates to a centrifugal master clutch for a vehicular drivetrain system and, more particularly, to a centrifugal master clutch having improved wear life and disengagement characteristics.
2. Description of the Related Art
Centrifugally operated friction clutches are well known in the art of vehicular drivetrain systems and typically include an input member driven by a prime mover, usually an electric motor or internal combustion engine, and weights rotatable with the input member which, upon rotation of the driving member, will move radially outwardly under the effect of centrifugal force to cause the input member to frictionally engage a driven output member. Automatically actuated centrifugal clutches employed with heavy-duty electromechanical highway line-haul truck transmissions include so-called centrifugal actuation modules that house the centrifugally actuated weights. The centrifugal modules are drivingly connected to an engine flywheel, and each of a plurality of centrifugally actuated weights is adapted to swing in an arc about a pivot link fixed to the module housing structure. As such, the so-called swing weights contained within the modules are radially outwardly movable against resistive spring forces as a function of engine speed—the higher the speed, the greater the outward movement between limits. Rollers attached to the weights are adapted to roll atop ramp segments that are cammed for clutch engagement and disengagement.
The swing weights are subjected to a number of forces, and thus give rise to issues that work against satisfactory operation of the modules over the useful lives of the clutch. For example, in one known centrifugal clutch, each of the swing weights is biased by its own compression spring(s). In this design, the biasing force exerted on a swing weights by its compression spring(s) is dependent on the position of the swing weight-generally the farther outward the swing weight moves, the greater the spring force exerted on the swing weight. As the friction materials in the clutch wear, the swing weights move farther up the ramp segments to create a given clamp load and the engagement point of the clutch undesirably changes due to the additional compression of the swing weight springs.
Another feature of the above-described prior art centrifugal clutch is the use of two different ramp surfaces on the ramp segments. A first ramp surface exhibits a relatively steep slope and a second ramp surface exhibits a more gradual slope. These ramp surfaces are engaged by swing weight rollers and are used to create a clamp load as the centrifugal force acting on each swing weight increases. Particularly, as the centrifugal force increases, the swing weights will move from their original position on the relatively steep first ramp surface onto the more gradual sloping second ramp surface. Since a centrifugal clutch operates as a balance of forces, any tolerance in the centrifugal module components (e.g., swing weight springs, ramp segments, etc.) may cause a “staggered disengagement”, wherein one or more of the swing weights moves from the second ramp surface to the first ramp surface before the other swing weights. This condition is exacerbated in a swing weight style centrifugal clutch since operation of each individual swing weight is essentially independent of the other swing weights.
Accordingly, a need exists for an improved centrifugal clutch that maintains the engagement point of the clutch and avoids staggered disengagement of the centrifugally operated weights.